Abstract Type 1 diabetes (T1D) is one of the most common chronic diseases in childhood, and the incidence has been increasing for several decades. Weight management in T1D has been underemphasized until recently, when it became recognized that the T1D population is not spared from the childhood obesity epidemic and is even more susceptible to the complications of obesity. Critically, being overweight in T1D is strongly associated with poor glycemic control, underscoring a need to simultaneously optimize these critical behaviorally-mediated outcomes. Weight and glycemic control are behaviorally linked through underlying self-regulation of food intake and activity. The challenge of weight control in T1D is the integration of weight-oriented self-regulation into the complex, existing T1D self-regulation in such a way that prevents adverse effects on glycemia. An approach to optimize weight alongside glycemic control is to capitalize on a universal challenge to the self-regulation that underlies both outcomes, such as impulsivity. Based on the demonstrated associations of impulsivity in youth with obesity and with worsened glycemic control in T1D, we hypothesize that impulsivity may predict eating behaviors in T1D that drive long-term increases in body mass index z-score (BMIz) and hemoglobin A1c (HbA1c). Based on recent models of disordered eating in T1D and new preliminary data, we further hypothesize that impulsive eating is prevalent in episodes of non-severe hypoglycemia, contributing to excess dietary intake and glycemic overcorrection that drives the long-term effects on weight and glycemia. In sum, impulsivity has potential as a target for T1D weight control interventions to increase self-regulation efficacy, which may also improve HbA1c. The objective of this Kirschstein-NRSA individual fellowship (F30) is to characterize impulsivity as a determinant of both BMIz and HbA1c in youth with T1D and explore the eating behaviors that may underlie this association. This proposal will use existing data from a subset of T1D youth (n=126, ages 12- 16, T1D duration>1 year, HbA1c=8-13%) from the Flexible Lifestyles Empowering Change trial (FL3X, NIH 1UC4DK101132). The specific aims of this proposal are to: 1) Elucidate the relationship of impulsivity with BMIz and HbA1c as independent and joint outcomes 2) Utilize data from continuous glucose monitoring to characterize how the frequency of non-severe hypoglycemia, overcorrection thereof, and general glycemic variability predict BMIz. In addition, we will test whether the effect is modified by impulsivity. Overall, the study of impulsivity, body weight, and glycemic control? enriched by novel investigation into effects of impulsivity on patterns of hypoglycemia and linked eating behavior?will open productive avenues for future interventions to improve weight management and glycemic control in T1D youth. Augmented by a rigorous curriculum in nutrition and epidemiology, a longitudinal endocrinology clerkship, and a diabetes clinic service project, this research is central to a comprehensive training plan for a translational career in nutrition science and medicine under Beth Mayer-Davis, PhD, John Buse, MD, PhD, and Kyle Burger, PhD.